Refiners can be used to mechanically process fiber containing materials such as biomass. Refiners come in a variety of different designs where each design is tailored for particular purposes. A common feature for quite a few of these designs is that fibrous material is fed through an inlet, such as a feeding channel, in the refiner to arrive at a refining area in which the material is processed, i.e. ground by means of refining blades. These refining blades also come with different designs and a variety of different geometrical shapes. The refining blades might for example be of a disc shape type where the refining surfaces of the refining blades extend more or less perpendicular to the inlet. One particular version of such a design comprises two axially aligned refining blades that are linearly displaced along a common axis that is more or less parallel with the material inlet. The grinding surfaces, or the refining surfaces, of the refining blades will in this particular design be facing each other. The area between the refining blades defines a refining gap. In a typical case one of the two refining blades will be attached to, for example, an end portion of a material feeding axis and comprise a centrally located through hole through which the material is fed into the refining area. Upon entrance into the grinding area the material will be brought into contact with the surfaces of the refining blades. The refining blades are provided in the refiner in such a way that they can be rotated around an axis. By rotating the refining blades the material present in the refining gap will be ground between the refining surfaces before leaving the grinding area. The described refining blades might be separately rotatable, often in such a way that they could be rotated in opposite directions. It is however also possible that only one of the refining blades is rotatable while the other remains fixed. In this particular design the rotating refining blade is referred to as a rotor while the fixed, static refining blade is referred to as a stator.
There also exist refiners where the refining blades have a conical shape. In this particular design two conically shaped refining blades are arranged coaxially around a common axis of rotational symmetry. The outer larger conically shaped refining blade has an inner surface opposing an outer surface of the inner smaller conically shaped refining blade. The gap between these opposing surfaces constitutes the refining gap and during operation the material to be processed is directed into this gap and ground between the two surfaces.
To obtain a more efficient grinding action, it is possible to provide the refining surfaces of the refining blades with bars. These bars consist of surface structures such as grooves placed side by side with protruding bars of metal or other hard materials of various geometrical shapes. The bars are provided to further increase the effectiveness of the refining and/or to provide certain specified desirable effects. A possible form of bars as used in a refiner with conically shaped rotors and stators is given in WO 2009/040477 A1. Another type of refining blades is disclosed in WO 2009/097963 A2. Both of these disclosures relate solely to choosing shapes for bars so as to obtain an effective refining of the pulp.
During operation of a refiner with conically shaped refining blades, fibrous material is fed through an inlet such as a feeding channel and thus enters the grinding area. Normally the material will enter the refining area in a direction that is more or less parallel to the symmetry axis of the conically shaped refining blades. The material is then directed into the area that define the refining gap between the grinding surfaces of the refining blades and brought into contact with the bars. Since the bars are more or less delicate surface structures protruding from the grinding surface they are slightly vulnerable to damages. It might for example be the case that the material fed into the refiner carries debris with it. In the case of dispersion of pulp the debris could, for example, be stones or steel remnants from the equipment used to cut the tree. It could also be remaining pieces of wire such as those used when packing the material into bales. Another common type of debris when dispersing pulp emanates from solid remnants in recycled cardboard or paper. This could for example be clips used in a cardboard box, but it could literally be any type of debris present in recycled cardboard or paper. All solid objects present in the material to be ground in the refiner could potentially damage the bars of the refining blades. Beside the fact that such debris in itself might severely damage the bars, there is also a potential risk that a bar of a refining blade, when damaged by debris, will be broken off from the refining blade and cause further damages to neighboring bars, thus creating a cascade of damaging debris that will cause further damages to the refining blades
A known measure taken in order to at least partially prohibit debris from entering a refiner is to wash the material before it is processed by the refiner. Even though this is a rather effective means to remove a substantial part of the debris there is still a risk that solid objects will enter the refiner. If, despite the washing, debris do enter the refiner it is usually relied upon gravity to prohibit the debris from contacting the refining blades. That is, the intention is that solid objects contained in the material even after washing will sink towards the bottom of the refining zone before the material is brought into contact with the refining blades. The effectiveness of this particular solution is however dependent upon the concentration of the material, that is the amount of water or fluid in the material mixture. It should be noted that even in applications where the concentration is rather low, that is, when the fibrous material contains a lot of water or some other fluids, the material might still carry debris with it and bring it into contact with the refining blades. This might, for example, be the case if the feeding velocity of the material is high whereby the debris is swiftly transported to the refiner blades before it has had time to sink to the bottom of the refining zone.
The refining blades with their corresponding surface structures, that is the bars, are in general quite expensive and delicate to construct and debris present in the pulp therefore constitute a nuisance within the technical field, a nuisance that might lead to severe refiner damages and, as a consequence, to expensive and prolonged shutdowns of the refining process. The present invention is designed to at least partially mitigate the problems related to debris damaging the refining blades of a refiner.